What is an electrocardiogram?

An electrocardiogram (ECG or EKG), is a measurement of the electrical activity of the heart. By placing electrodes at specific locations on the body (chest, arms, and legs), a graphic representation, or tracing, of the electrical activity can be obtained. Changes in an ECG from the normal tracing can indicate one or more of several heart-related conditions. Disorders that are not associated with heart conditions may also cause changes in the ECG.

To better understand the ECG, it is helpful to understand the heart's electrical conduction system.

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The heart's electrical system

The heart is, in the simplest terms, a pump made up of muscle tissue. Like all pumps, the heart requires a source of energy and oxygen in order to function. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.

How does the heart beat?

An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber) of the heart. The sinus node generates an electrical stimulus regularly (60–100 times per minute under normal conditions) and is sometimes referred to as the "pacemaker" of the heart. This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's lower chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart).

The electrical impulse travels from the sinus node to the atrioventricular node (also called AV node), where impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to the right and left ventricles.

Normally at rest, as the electrical impulse moves through the heart, the heart contracts about 60 to 100 times a minute. Each contraction of the ventricles represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.

Almost all heart tissue, under certain conditions, is capable of starting a heartbeat, or becoming a pacemaker.

The heart's natural pacemaker develops an abnormal rate or rhythm

The normal conduction pathway is interrupted

Another part of the heart takes over as pacemaker

What does an ECG mean?

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Almost everyone knows what a basic ECG tracing looks like. But what does it mean?

The first little upward notch of the ECG tracing is called the "P wave." The P wave indicates that the atria (the two upper chambers of the heart) are electrically stimulated to pump blood to the ventricles.

The next part of the tracing is a short downward section connected to a tall upward section. This next part is called the "QRS complex." This part indicates that the ventricles (the two lower chambers of the heart) are electrically stimulated to pump out blood.

The next short flat segment is called the "ST segment." The ST segment indicates the amount of time from the end of the contraction of the ventricles to the beginning of the "T wave".

The next upward curve is the T wave. The T wave indicates the recovery period of the ventricles after contraction.

When your physician studies your ECG, he or she looks at the size and length of each part of the ECG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12-lead ECG will look different, but will have the same basic components as described above. Each lead of the 12-lead is "looking" at a specific part of the heart, so variations in a lead may indicate a problem with the part of the heart associated with the lead.

Why is an ECG done?

Many conditions can cause changes to the ECG. Because the ECG is a fast, simple, painless and relatively inexpensive test, it may be used as a part of an initial examination to help the physician narrow the scope of the diagnostic process. ECGs are also done with routine physical examinations so that comparisons can be made with previous ECGs to determine if a hidden or undetected condition might be causing changes in the ECG. Some conditions which may cause changes in the ECG pattern may include, but are not limited to, the following:

Ischemia. Decreased flow of oxygenated blood to the heart due to obstruction in an artery.

Heart attack. Also called myocardial infarction (MI); damage to the heart muscle due to insufficient blood supply.

Conduction disorders. A dysfunction in the heart's electrical conduction system, which can make the heartbeat too fast, too slow, or at an uneven rate.

Electrolyte disturbances. An imbalance in the level of electrolytes, or chemicals, in the blood, such as potassium, magnesium, or calcium.

Pericarditis. An inflammation of the sac (thin covering) that surrounds the heart.

Valvular heart disease. One or more of the heart's four valves can become defective, or may be congenitally malformed at birth.

Enlarged heart. An abnormally large heart can be caused by various factors, such as valve disorders, high blood pressure, congestive heart failure, electrical disturbances, or congenital (present at birth) abnormalities.

Chest trauma. Blunt trauma to the chest, such as a motorist hitting the steering wheel in an automobile accident.

This list is presented as an example. It is not intended to be a comprehensive list of all conditions which may cause changes in the ECG pattern.

An ECG may also be done for the following reasons:

To obtain a baseline tracing of the heart's function (during a physical examination). This baseline tracing may be used later as a comparison with future ECGs, to see if any changes have occurred.

As part of a work-up prior to a procedure such as surgery to make sure a heart condition does not exist that might cause complications during or after the procedure

To check the function of an implanted pacemaker

To check the effectiveness of certain heart medications

To check the heart's status after an MI, or after a heart-related procedure such as a cardiac catheterization, heart surgery, electrophysiological studies, etc.

How is an ECG done?

An ECG is one of the simplest and fastest procedures used to evaluate the heart. An ECG technician, nurse, or physician typically will place 12 separate electrodes (small plastic patches) at specific locations on your chest, arms, and legs. The electrodes are self-sticking and will adhere to the skin. The area where the electrodes are placed may be cleaned, or hair may need to be shaved or clipped so there is a better connection. You will be lying down on a stretcher or bed, and the leads (wires) will be connected to the electrodes. You will need to lie very still and not talk during the ECG procedure, as movement or talking may interfere with the tracing. The technician, nurse, or physician will start the tracing, which will take just a few minutes. You will not feel anything during the tracing. Once a clear tracing has been obtained, the leads and electrodes will be removed, and you will be free to continue on with your usual activities, unless directed otherwise by your physician. An ECG can indicate the presence of arrhythmias (an abnormal rhythm of the heart), damage to the heart caused by ischemia (lack of oxygen to the heart muscle) or myocardial infarction (MI, or heart attack), a problem with one or more of the heart valves, or other types of heart conditions.

There are additional ECG procedures which are more involved than the basic ECG. These procedures include the following:

Exercise ECG, or stress test. The patient is attached to the ECG machine as described above. However, rather than lying down, the patient exercises by walking on a treadmill or pedaling a stationary bicycle while the ECG is recorded. This test is done to assess changes in the ECG during stress such as exercise.

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Signal-averaged ECG. This procedure is done in the same manner as a resting ECG, except that the heart's electrical activity is recorded over a longer period of time, usually 15 to 20 minutes. Signal-averaged ECG is done when risk for arrhythmia is suspected, to detect subtle abnormalities in the ECG that are not visible to the naked eye.

Holter monitor. A Holter monitor is an ECG recording done over a period of 24 or more hours. Three electrodes are attached to the patient's chest and connected to a small portable ECG recorder by lead wires. The patient goes about his or her usual daily activities (except for activities such as taking a shower, swimming, or any activity causing an excessive amount of sweating which would cause the electrodes to become loose or fall off) during this procedure. There are 2 types of Holter monitoring:

Continuous recording. The ECG is recorded continuously during the entire testing period.

Event monitor, or loop recording. The ECG is recorded only when the patient starts the recording, when symptoms are felt.

Holter monitoring may be done when arrhythmia is suspected but not seen on a resting or signal-average ECG, since arrhythmias may be transient in nature and not seen during the shorter recording times of the resting or signal-average ECG.